The present invention relates to a variable magnification type copying machine using a zoom lens, and more particularly to a mechanism for moving the movable lens components in the zoom lens.
FIG. 1 shows, from top to bottom, the positions of a movable front lens group L1 and a movable rear lens group L2 of a zoom lens in the direction of the optical axis when the magnification is larger than one (enlargement), one (life size) and smaller than one (reduction), and the loci of the two lens groups in moving between these positions, wherein the original platen D and the photosensitive surfaces are stationary, that is, with the distance between the object and the image being held unchanged.
FIGS. 2 and 3 show an example of the construction of a conventional copying machine magnification varying device which is used to move the front lens group L1 and the rear lens group L2 along the loci shown in FIG. 1 above. In FIG. 2, the original platen D, the front lens group L1 and the rear lens group L2 are the same as those in FIG. 1, and the photosensitive surface S is a photosensitive drum. A full-speed mirror M1, arranged below the original platen D and made of a transparent glass plate, runs together with an illuminating lamp R1. Furthermore, optical path changing mirrors M2 and M3 for receiving a light beam reflected from the full-speed mirror M1 are supported below the original platen D and moved as one unit. In order to maintain the length of the optical path unchanged, the mirrors M2 and M3 are moved in the same direction as the full-speed mirror M1 and at half of the speed of the full-speed mirror M1. For this reason, the mirrors M2 and M3 in combination are called "half-speed" mirrors. The optical path changing mirrors M2 and M3 reflect the light beam from the full-speed mirror M1 so that the light beam is applied through the zoom lens L to a fourth mirror M4. The fourth mirror M4 applies the light beam to the photosensitive drum S, which rotates in synchronization with the scanning of the full speed mirror M1 (speed of the full-speed mirror M1=(1/magnification).times.peripheral speed of the drum S). In order to change the magnification, the movable lens groups L1 and L2 of the zoom lens are moved along the optical axis and the distance between the two lens groups is set as shown in FIG. 1.
FIG. 3 shows an example of a conventional mechanism for moving the movable lens groups L1 and L2. In the lens moving mechanism, the force of movement of the front lens group L1 is utilized to move the rear lens group L2 through a cam. In FIG. 3, reference character F designates the optical axis of the zoom lens L, and 11, a guide bar which is fixedly mounted on supporting brackets 13 provided on a base plate 12 and extends parallel to the optical axis F. A zoom lens mounting frame 14 is slidably mounted on the guide bar 11. Guide rollers 15 at one end of the zoom lens mounting frame 14 roll on a guide rail 16 which is fixed on the base plate 12 and is arranged parallel to the guide bar 11.
A first lens barrel 17 supporting the front lens group L1 is fixed to the zoom lens mounting frame 14. A second lens barrel 18 is supporting the rear lens group L2 is slidably arranged on the first lens barrel 17 in such a manner that the optical axes of the two lens groups are in alignment with each other. With respect to the zoom lens mounting frame 14, the first lens barrel 17 is stationary while the second lens barrel 18 is movable. An arm 21 extends from one end of the zoom lens mounting frame 14. A feed screw 22 is supported by the supporting brackets 13 and extends parallel to the guide bar 11. The feed screw 22 thus supported is threadedly engaged with the arm 21 extending from the mounting frame 14. The feed screw 22 is turned by a stepping motor (or a pulse motor) 23 whose angular position is controllable via a timing pulley 24, a timing belt 25 and a timing pulley 26.
A shaft 28 is supported by the zoom lens mounting frame 14 and is mounted perpendicular to the optical axis. A distance adjusting member, namely a cam plate 29, is rotatably mounted on the shaft 28. A cam follower 30 protrudes from the second lens barrel 18 and extends to a point where it contacts the peripheral cam surface of the cam plate 29. A tension spring 31 is provided to maintain the cam plate 29 in contact with the cam follower 30. When the cam plate 29 rotates, the second lens barrel 18 is moved along the optical axis F along a locus determined according to the configuration of the cam surface of the cam plate 29. The cam plate 29 is shaped so that the rear lens group L2 supported by the second lens barrel 18 is moved along the locus G shown in FIG. 1.
A wire driving pulley 33 is mounted on the shaft of the cam plate 29. The wire driving pulley 33 is movable relative to the cam plate 29 so as to adjust the relative rotational phase therebetween. The wire driving pulley 33 is fixedly mounted on the shaft of the cam plate 29 after phase adjustment. A wire 35 is secured to the wire driving pulley 33 with a small screw or the like and is then wound on the wire driving pulley 33. The two ends of the wire 35 are fastened to respective ones of the supporting brackets 13.
In the conventional device thus constructed, as the stepping motor 23 rotates the feed screw 22, the zoom lens mounting frame 14 is moved along the guide bar 11 so that the zoom lens L, including the front lens group L1 and the rear lens group L2, is moved in the direction of the optical axis F. In this operation, the zoom lens mounting frame 14 describes a locus corresponding to the locus E of the front lens group L1 in FIG. 1 with the distance of movement depending on the copying magnification.
The middle part of the wire 35, both ends of which are fixedly held, is secured to the wire driving pulley 33 so that the wire will not slide on the wire driving pulley. Therefore, as the zoom lens mounting frame 14 is moved, the wire driving pulley 33, and accordingly the cam plate 29 coaxial with the wire driving pulley, rotates through an angle corresponding to the amount of movement of the zoom lens mounting frame 14. Therefore, the second lens barrel 18 is moved in conformance with the configuration of the cam plate 29 through the cam follower 30, which is elastically in contact wih the former, so that the distance between the front lens group L1 and the rear lens group L2 is set to a value corresponding to the required magnification, that is, the rear lens group is moved along the path G in FIG. 1.
The above-described conventional device suffers from the following difficulties: Since the cam plate 29, the wire driving pulley 30, etc., must be arranged beside or near the lens in such a manner that they are perpendicular to the optical axis of the lens, the device is necessarily bulky. Furthermore, as the lens barrels 17 and 18 for the front and rear lens groups L1 and L2 and the relevant components are intricate in construction, the manufacturing cost is relatively high. Still further, because the wire 33 is low in dimensional accuracy, it is essential to make a phase adjustment to the wire driving pulley 33 and the cam 29 in the assembly process. That is, assembling the device is rather difficult. As the cam plate 29 is disc shaped, the diameter of the wire driving pulley 33 must be such that the wire driving pulley 33 makes substantially one revolution for the total amount of movement of the front lens group L1. Accordingly, if it is required to increase the total amount of movement, it is necessary to increase the diameter of the pulley, and thus it is necessary to increase the height of the device.